Transmitters for electromagnetic communication systems



0. B. SNEATH Aug. 8, 1961 TRANSMITTERS FOR ELECTROMAGNETIC COMMUNICATIONSYSTEMS Filed Dec. 16, 1957 FIG. 2.

This invention relates to transmitters for electromagnetic communicationsystems whereby individuals can be selectively called by transmittingsignals on a correspondingly selected frequency within the audio andintermediate frequency range.

The invention has for an object to provide simpler apparatus and tosimplify the procedure for calling one of a number of persons.

A further object is to provide means whereby a call can be sent out bypressing either one or two keys of a press button or piano key switch ofa type commonly used in radio receivers. In this type of switch thepressing of a key causes any key already pressed to be re leased. Thekey pressed remains down until further keys are pressed. It is however,possible in such switches to lock down two keys at once by pressing themdown simultaneously.

It will be seen that, for example, with eight keys, eight settings arepossible with a single key depressed and 28 settings are possible withtwo keys depressed, making a total of 36 possible call numbers. With tenkeys the number of possible call numbers is 55 and with 12 keys thenumber of possible call numbers is 78.

Considering the 8-key arrangement, the possible combinations arerepresented by two digit numbers. It will be apparent that under thissystem the call numbers 84 and 48 would give rise to the depression ofthe same keys and would therefore be identical. It is preferred,therefore, that only numbers in which the first digit is less than thesecond be used as call numbers.

The general principle of working is such that if more than one number ispressed, the higher number selects a frequency range within which thefrequency is to fall, while the lower number selects a frequency withinthat range, as will appear from the following description: The order ofthe different frequencies may be the same as or the inverse of the orderof the numbers or they may be otherwise related. A single number will belocated as if it were a double number of the same digit, i.e. fourrepresents 44 and six represents 66.

In general, it is only required that a call should go out for a shorttime. Accordingly the pressing of any key or button may be made tooperate a timing circuit which starts the call which is then terminatedautomatically. The key or buttons will remain depressed and willtherefore indicate the last call sent until another key or button ispressed for another call. If it is desired to repeat the call for whichthe keys or buttons are already depressed, this may be done by pressinga repeat key or button or alternatively by pressing momentarily harddown beyond their locking position the keys already depressed.

Such an arrangement has the advantage over a system of independentswitches that it is never necessary to return to off position theswitches previously operated, before making a new call.

In switches of the type above referred to, a locking or sliding bar isdisplaced when a key is depressed, and returns whole or part of the wayto the original position when the key is locked. The movement of thisbar can be utilised to operate micro-switches or normal switches so thatcircuits are made temporarily to initiate the call. The switches arethus virtually ganged together by this Patented Aug. 8, 1961 "ice bar.As an alternative method, each key operated bar can be made to operate amake-before-break contacts. By paralleling the make contacts and thebreak contacts respectively on all the keys, leaving the change-overcontact unconnected, the pressing of any key will cause a momentarymaking of contacts, which can be used to initiate the call. It isdesirable that there should be a delay of something of the order of aquarter of a second between the making of the contacts and the startingof the call to ensure that both the keys pressed are in position and allprevious keys have been returned before the call starts.

Embodiments will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a circuit diagram of a frequency selecting arrangementaccording to the invention,

FIG. 2 is a circuit diagram illustrating a modified form of part of thearrangement shown in FIG. 1 from the point X therein.

In the preferred arrangement according to the invention as illustratedin FIG. 1, a main switch 1A and subsidiary switches 1B, 1C, ID areganged together for operation by the number 1 key, a main switch 2A andsubsidiary switches 2B, 2C, 2D are ganged together for operation by thenumber 2 key and so on up to key number 8. The keys themselves are notshown in the circuit diagram. The frequency range is selected by thehighest number key depressed by virtue of its selecting the tap on aninductor L to which tuning condensers C and C are connected. One leadfrom the condenser C is connected to the end of L and the other leadfrom the condenser is taken to the moving contact of the main switch 8aon the highest number key. If the key is pressed the condenser C isconnected to the first tap on the inductor L If the key is not pressed,the condenser is switched through to the moving contact 7a of the nextkey. If this main key is pressed, connection is made to the next tap onthe inductor L If this key is not pressed, however, contact is made tothe number 6 key, and so on. Accordingly, the tap to which the condenseris connected is determined by the highest number key pressed.

The frequency within the range is selected by the lower number keydepressed in that it shunts the whole or part of the main inductance Lwith a subsidiary inductance L preferably of higher value. For thispurpose, a lead 1 from a tap T or T on the main inductor, which tap isselected by the subsidiary switches IE to 8B, is taken to a movingcontact 10 operated by the number 1 key. If this key is pressed the leadis connected nowhere. If the number 1 key is not pressed, the lead 1 isconnected to the moving contact of subsidiary switch 2C operated by thenumber 2 key, and if this is pressed the lead 1 is connected to thehighest impedance tap on the subsidiary inductance L If the number 2 keyis not pressed, it connects the lead to the moving contact on the number3 key which, if pressed, connects it to the next tap on the inductanceand so on, all as illustrated in FIG. 1.

It is of course possible to have modified arrangements which make allconnections to the keys in the reverse or different orders to achievethe same eifect.

The frequency selection is illustrated in FIG. 1, where keys 2 and 5 areshown depressed and the coils are shown coupled to a valve V1 tomaintain oscillation. It will be seen that the point on the maininductance L to which the auxiliary inductance L is connected, is variedaccording to whether the highest number key pressed is greater or notthan 4. If the key is greater than 4, the part of the main inductanceacross which the auxiliary inductance is shunted will be smaller. Theresult of this is that the percentage gap between adjacent frequenciesis less at higher frequencies. In the case of a 36 frequency transmitterwith frequencies between 2,000 and 14,000 cycles per second, the spacingbetween adjacent channels might be 9 percent at the lower end andpercent at the higher end.

The frequency of any arrangement may, of course, be calculated from theinductance and capacity of the components. The chief uncertainty in thecalculation being the stray capacity of the windings and circuit. Amethod of avoiding this to some extent is to calculate on the basis of acapacity across the main inductance greater than the stray capacity andto insert a trimming condenser C permanently across the whole or somepart of the main inductance. The trimming condenser can then be adjustedto bring the stray capacity up to the required value.

The oscillator circuit described above may be coupled to an output stage50 in various manners, for example by condenser C50.

The higher number, or single, key pressed may also serve to bring intothe circuit one of a plurality of condensers C to C which at thefrequency of the signal will constitute a series resonance with the loopL or, where a matching transformer is employed, with the reflectedimpedance of the loop through the transformer.

An alternative arrangement employing a similar switching system isillustrated in FIG. 2. Here the lower number pressed changes a capacityinstead of shunting the main inductance with an auxiliary inductance.FIG. 2 shows a modification of the circuit of FIG. 1 consisting ofreplacing the coil L and the section of the switches 1C to SC in thatcircuit from the pointmarked X.

It is, of course, not necessary for all possible combinations of thenumbers to be available for use. Thus, only keys 1, 2 and 3 of eightkeys could be caused to operate contacts for determining a frequencywithin a range. All keys higher than 3 could then operate the samecontact as 3. This would render the following distinct combinationsavailable:

I claim:

1. In a transmitter for an electromagnetic communication system, meansproviding for the selection of a desired frequency of transmitted signalfrom a single oscillator, said means comprising an inductance, aplurality of tappings on said inductance, a main two-way switch for eachsaid tapping, each said main switch except the last having a movingcontact normally connected to one pole of the next switch of which theother pole is connected to its corresponding tapping, a tuning capacitorfor said inductance connected to the moving contact of said last switch,whereby operation of the switch nearest that end connects said tuningcapacitor to the corresponding tapping, whereby to select a frequencyrange, subsidiary tuning means for said inductance constructed andarranged to change the resonant frequency of the circuit branchcomprising said inductance and tuning capacitor when the subsidiarytuning means is connected to said inductance, a plurality of leadsconnected respectively to tapped portions of said subsidiary tuningmeans, a subsidiary two-way switch for each said lead, each saidsubsidiary switch except the first having a moving con tact normallyconnected to one pole of the preceding subsidiary switch of which theother pole is connected to its corresponding lead, the moving contact ofsaid first subsidiary switch being connected to said inductance, wherebyoperation of the subsidiary switch nearest said first subsidiary switchconnects said subsidiary tuning means to said inductance, whereby toselect a specific frequency within said range, each said subsidiaryswitch being ganged with a corresponding main switch for operationtherewith.

2. A frequency selector as claimed in claim 1, wherein said subsidiarytuning means comprises a subsidiary inductance and wherein the switchesare so arranged that upon operation of two switches, the switch nearerto one side determines the tap on the first-named inductance, and theswitch nearer to the'other side determines the tap on the subsidiaryinductance, the frequency of the signal transmitted being determined bythe combination of the two inductances.

3. A frequency selector as claimed in claim 1, wherein said subsidiarytuning means comprises a capacitance, and wherein the switches are soarranged that upon operation of two switches, the switch nearer to oneside determines the tap on the inductance and the switch nearer to theother side determines the tap on the capacitance and the combination ofthe inductance and the capacitance determines the resonant frequency ofsaid circuit branch.

4. A frequency selector as claimed in claim 1, wherein the values of theinductance and of the tuning capacitor and subsidiary tuning means areso arranged that the switch nearer to one side determines which ofseveral ranges of adjacent frequencies is used, and the other switchdetermines the frequencies within this range.

5. A frequency selector as claimed in claim 4, and means whereby theswitch which determines which range of frequencies is selected alsochanges the tap on the firstnamed inductance to which the subsidiarytuning means is connected, so as to adjust the spacing of thefrequencies within the range to the most suitable values for this range.1

References Cited in the file of this patent UNITED STATES PATENTS2,122,183 Schwarzer June 28, 1938 2,295,173 Hofi'mann Sept. 8, 19422,354,148 Shaw July 18, 1944 2,505,182 Haller et a1 Apr. 25, 19502,522,973 Turner Sept. 19, 1950 2,581,159 Achenbach Jan. 1, 19522,806,954 Tennes Sept. 17, 1957

